Various types of memory storage devices typically utilize a sense amplifier to amplify the voltage or current level stored in the individual memory cells. This voltage or current level represents the logic state stored in the memory cells. Static random access memory ("SRAM") devices store data in a cell comprising a flip-flop which is typically made up of six transistors. On the other hand, dynamic random access memory ("DRAM") devices store data in a cell comprising a small value capacitor and a transistor.
Regardless of the type or technology of memory, usually the voltage or current level signal stored in a memory cell is a very small value. The sense amplifier must amplify this normally severely attenuated signal. Further, since such signals can be of a level that is much smaller that undesired signals picked up by crosstalk from other parts of the memory device, the sense amplifier generally must be capable of rejecting such noise signals. As a result, prior art sense amplifiers typically have been differential amplifiers having a relatively high common-mode rejection capability. This is because picked-up interference is usually a common-mode signal. However, differential amplifiers are useful only with memories having complementary dual outputs. They are not useful for memories having a single-ended output.
Another consideration with the design of sense amplifiers is that of power dissipation and speed of operation. Fully static RAMs are those in which the sense amplifiers are turned on all the time. The problem with this is that the power dissipation level is relatively high and the speed relatively slow. To reduce the power dissipation level and increase the speed, some SRAMs use sense amplifiers that are normally turned off and activated only when needed to carry out the sensing function.